Sampling pipettes are known from the prior art having a conventional design of the type integrating an upper pipette body forming a handle, and a lower pipette body having at its lower end one or more tip holding nozzles, whose function is to hold sampling tips, also called consumables. The lower pipette body houses a sliding piston controlled by manual or motorized equipment causing the piston to move upward during liquid sampling phases and to move downward during liquid transfer phases. The upward movement is generally performed under the effect of release of a spring that is compressed during the previous downward movement. This type of design is found both in single channel pipettes, having a single tip holding nozzle, and in multichannel pipettes, having a plurality of tip holding nozzles, whether the pipette is manual or motorized.
The upward stroke imposed upon the piston determines the volume of sampled liquid, a volume which is previously set by the user using a variety of possible controls including a thumb wheel, an adjusting screw, a digital keypad, etc.
On conventional pipettes, the piston is of strictly cylindrical shape and slides within a cavity of complementary shape, made in the lower body of the pipette and delimiting an aspiration chamber. The aspiration chamber is partly delimited by the lower end of the piston, which means that its volume varies when the piston is moved. Therefore, the volume of sampled liquid, corresponding to the increase in air volume in the aspiration chamber subsequent to a given stroke of the piston, is substantially equal to the product of the cross-section of the piston by the length of the given stroke of the piston.
Consequently, the sampling capacity of a pipette is determined at the present time both by the cross-section of the piston and by the length of the maximum stroke. Therefore, to increase pipette capacity in terms of the maximum value of liquid volume the pipette can sample or the ratio between the maximum and minimum values the liquid volume pipette can sample, typically in the order of 10 to 20, it is necessary to increase the value of at least one of the cross-section of the piston and/or the length of the piston stroke. Relative to the maximum stroke length, any increase in length rapidly leads to problems of global ergonomics for the pipette. Relative to the cross-section of the piston, i.e. the piston diameter, any increase thereof generally has a detrimental effect on the accuracy and repeatability of the sampled volume. The design of conventional pipettes does not therefore allow the simultaneous combining of essential criteria such as a large sampling capacity, ergonomics, accuracy, and repeatability of sampled volumes.
Multi-volume pipettes include a succession of chambers of increasing diameters/volumes starting from the tip holder, each one cooperating with a piston section of corresponding diameter. The placing or non-placing in communication of these chambers, isolated from each other, allows the pipette to be adapted to the value of the liquid volume to be sampled. Nevertheless, multi-volume pipettes do not solve the problems in a fully satisfactory manner because the more the capacity of the pipette is increased, the greater the number of aspiration chambers that are superimposed in the direction of the piston's sliding movement. The increase in the number of chambers leads to an increase in the total length of the pipette which is detrimental to the pipette's ergonomics. Also, the greater the volume of liquid to be sampled, the less accuracy and repeatable the pipette becomes due to the chamber and piston having a greater diameter.